Synergetic effect in a self-doping polyaniline/TiO₂ composite for selective adsorption of heavy metal ions

Design and synthesis of adsorbents with high adsorption selectivity are important for the treatment of the wastewater which contains multiple metal ions. Herein, a PANi(ES⁺)/TiO₂(O⁻) composite is the subject of an investigation into the synergetic effect of a polymer/metal oxide composite and the mechanism of selective adsorption towards heavy metal ions. The self-doping nature of TiO₂(O⁻) to PANi was first discovered through characterizations including FT-IR, zeta potential analysis, TGA, XRD, SEM-EDS, TEM-EDS, N₂ adsorption-desorption isotherm and isotherm investigation for Pb (II), Zn (II) and Cu (II) in single/multiple metal ion system, which was confirmed to contribute to the novel selectivity of the PANi/TiO₂ composite of Zn²⁺>Pb²⁺>>Cu²⁺ in a multiple metal ion system. The synergistic mechanism between the conjugated polymer and metal oxide for selective adsorption was discussed and proposed subsequently, which was suggested that the metastability of the doped state of PANi (ES⁺) would result in the dedoping of the TiO₂(O⁻) in a non-acid solution (pH = 5), further leading to the doping of cationic heavy metal ions on TiO₂(O⁻). The mechanism we proposed can satisfactorily explain the selective adsorption properties of the polymer/metal oxide composite system and provide general guidance for designing an adsorbent with expected selective adsorption properties for water treatment.